Concrete samples in the laboratory

Concrete made from furnace cast-offs can withstand erosion

Image credit: Drexel University

Researchers in Philadelphia have developed a new recipe for concrete which uses recycled materials and resists chemical erosion due to salt grit.

The salt grit that is thrown over roads during snowy winters to ensure better road safety is gradually eroding the roads themselves, leading to extensive patching and repairing of roads every year.

The calcium chloride salt reacts with the calcium hydroxide in concrete to form a chemical by-product, calcium oxychloride, which expands in pores of cement when it forms. This causes roads to crumble.

“Many departments of transportation have reduced the amount of calcium chloride they used to melt ice and snow, even though it is very efficient at doing so, because it has also been found to be very destructive,” said Dr Yaghoob Farnam, a civil engineer and director of the advanced and sustainable infrastructure materials research group at Drexel University.

Dr Farnam has made it his goal to produce a concrete mix as strong as the ones currently used in roads, but which contain less reactive calcium hydroxide. To achieve this, he has turned to using cast-off products from furnaces and smelting.

Adding these recycled products, he hypothesised, would create a concrete mix that is more durable and does not react with road salt.

“There is a great push to use these power industry by-products because they take up space and some of them can be harmful to the environment,” said Dr Farnam. “We believed that portions of the byproducts such as fly ash, flag and silica fume could be used to make concrete that is both durable and cheaper, because it uses recycled materials.”

Dr Farnam’s team created cement samples using varying amounts of fly ash, silica fume and slag, and compared these with samples of Portland cement, the most common type used in American roads. They found that the samples containing the recycled materials did not produce as much calcium oxychloride, and suffered no damage during the testing period.

“This research proves that by using alternate cementitious materials to make concrete, they can avoid the destructive chemical reaction and continue to use calcium chloride.”

Dr Farnam is continuing to search for ways to improve the materials in our infrastructure, including by protecting concrete by adding a protective layer of bacteria to prevent calcium oxychloride formation.

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